RFID (radio frequency identification) systems have been known as one type of wireless communication systems. Such an RFID system generally includes a wireless frequency tag (also referred to as an “RFID tag”) and a reader-writer (RW) apparatus, wherein information is read from or written into the wireless frequency tag from the RW apparatus by means of wireless communication.
Known wireless frequency tags include one type of tags that can operate using a power source embedded in the wireless frequency tag (such a type is referred to as “active tags”) and another type of tags that operate using wireless waves received from an RW apparatus as driving power (such a type is referred to as “passive tags”).
In an RFID system using a passive tag, the wireless frequency tag operates an embedded integrated circuit, such as an IC or an LSI, using wireless signals from an RW apparatus as driving power, and performs various processing in accordance with received wireless signals (control signals). Transmission from the wireless frequency tag to the RW apparatus is achieved using reflected waves of the received wireless signals. That is, various information, such as a tag ID or results of the processing, is carried on the reflected waves, which is sent to the RW apparatus.
Note that a variety of frequency bands have been used for RFID systems, and recently, the UHF band (860 MHz to 960 MHz) is attracting attentions. The UHF band can allow long distance communications more easily than the 13.56 MHz band or the 2.45 GHz band that have been conventionally used. Frequencies around 868 MHz, 915 MHz, and 953 MHz are used in Europe in the United States, and in Japan, respectively. The communication ranges of wireless frequency tags (hereinafter, simply referred to as “tags”) in the UHF band are about between 3 meters and 5 meters, although the ranges depend on an integrated circuit, such as an IC chip or an LSI, used in the tags. In addition, the outputs of RW apparatuses are about one watt (W).
Conventional wireless frequency tags include, for example, those disclosed in Patent References 1 to 3 that will be listed below.
The technique disclosed in Patent Reference 1 is directed to reduce a drop in the communication distance of an RFID tag even when the RFID tag is used in proximity to a wireless absorptive material, thereby assuring reliability of communication. For this purpose, Patent Reference 1 discloses an RFID tag that includes a dielectric member shaped in the rectangular parallelepiped shape and having a predetermined permittivity, an antenna pattern for sending and receiving which is formed in a loop shape by means of etching or the like on the front face of this dielectric member, and an IC chip that is electrically connected to this antenna pattern via a chip-mounted pad.
When this RFID tag is used for an object that has a certain electrical conductivity, such as a bottle containing liquid or a living human body, a miniature loop antenna is formed by the antenna pattern around the dielectric member, which results in formation of a current loop on the object to which the tag is to be adhered. Thus, an even greater current loop is formed, which can contribute to increase in the gain of loop antenna, thereby increasing the communication distance.
The technique disclosed in Patent Reference 2 is directed to manufacturing an RFID tag that has a longer communication distance and that facilitates printing thereon. For this purpose, the RFID tag of Patent Reference 2 is formed by bonding a first component and a second component. The first component includes a plate-shaped first base made of a dielectric material, and a metal layer covering a first face of the front and back faces of the first base. The second component includes a sheet-like second base, and metal pattern that is formed on the second base and is electrically connected to the metal layer of the first component, forming a communication antenna, and a circuit chip that is connected to the metal pattern and performs wireless communication by the communication antenna, and a bonding material layer for bonding the second base to the second face opposing to the first face of the front and back faces of the first base. The metal layer of the first component and the metal pattern of the second component are electrically connected via a conducting component.
The technique disclosed in Patent Reference 3 is directed to providing an RFID tag that restrains a change in the resonant wavelength and the Q value, thereby assuring satisfactory communication status, even when the tag is disposed inside an apparatus including metal. For this purpose, Patent Reference 3 teaches a tag that is formed from a substrate in a substantial circular shape which has a loop-shaped antenna pattern and an IC, and a disk-shaped magnetic sheet which has a diameter substantially equal to that of the substrate, wherein the inductance can be easily adjusted by providing a cut-out portion of a single like in a part of the circumference of the magnetic sheet.
The magnetic sheet can reduce the influence of any metallic member disposed inside an apparatus. In addition, by selecting the width of the cut-out portion such that the reduction in the inductance of the antenna caused by the metal is offset by an increase in the inductance provided by the magnetic sheet, a change in the resonant wavelength and the Q value can be compensated, thereby assuring satisfactory communication status.
Patent Document 1: Japanese Patent Publication No. 2006-53833
Patent Document 2: Japanese Patent Publication No. 2006-301690
Patent Document 3: Japanese Patent Publication No. 2006-331101
When a wireless frequency tag for the UHF band is attached to metal, impedance matching with an integrated circuit, such as IC chips or LSIs (hereinafter, simply referred to as “chips”), and the gain may be deteriorated, which may render communication difficult. To address this issue, although various attempts have been made to form antenna patterns of wireless frequency tags in a loop shape, as the techniques disclosed in the above-referenced Patent References 1-3, it is difficult to adjust impedance matching (hereinafter, referred to as “matching adjustment”) in wireless frequency tags having a loop-shaped antenna pattern when the susceptance component of the chip (the imaginary part of the admittance which is the inverse of the impedance (typically, represented by B)) is significant.
That is, since the equivalent circuit of a chip mounted in a wireless frequency tag can be represented by the parallel capacitance component Ccp and the parallel resistance component Rcp, the susceptance component B varies dominantly dependent on the capacitance component Ccp. If the capacitance component Ccp becomes too high, design and adjustment of the antenna impedance to be matched with the capacitance component Ccp becomes difficult.
For example, one technique to adjust the antenna impedance is increasing the corresponding capacitance component Ccp by modifying (reducing) the relative permittivity of a dielectric material (substrate) on which the antenna pattern is formed, as depicted in FIG. 13 (relative permittivity versus Ccp characteristics). Since reduction in the relative permittivity is limited to a certain level (the minimum value is one which is the relative permittivity of air), however, it is difficult to address to a chip requiring the corresponding capacitance component Ccp of smaller than this limit (ES2 in FIG. 13).
In addition, although matching adjustment may be achieved by modifying the total length of a loop of a loop-shaped antenna pattern, the gain is reduced when the total length of the loop is shortened.